A hydraulic brake which is applied to vehicles at present includes a hydraulic piston and a caliper housing which includes a hydraulic cylinder. The hydraulic piston pushes and clamps a pad in a direction toward a disc.
Electro mechanical brakes (EMBs) clamp a pad by using driving parts which includes an electric motor, a reducer, and a screw, instead of a hydraulic cylinder and a hydraulic piston of a hydraulic brake. In the EMBs, a time taken in generating a maximum clamping force which is the same as that of hydraulic brakes is shorter than the hydraulic brakes. Also, since the EMBs have a fast response time, a braking distance is shorter in the EMBs than hydraulic brakes.
The EMBs can be categorized into direct-clamping type EMBs, wedge type EMBs (hereinafter referred to as an electro wedge brake (EWB)) using a wedge structure, and a differential gear type EMB using a differential gear, based on a type of a driving mechanism for pushing a pad.
Comparing the EMBs by types, the direct-clamping type EMBs are easy to implement than other types. On the other hand, in comparison with the direct-clamping type EMBs, the EWBs generate the same clamping force by using a motor having a small power, due to a self-reinforcing effect.
Moreover, in comparison with the direct-clamping type EMBs or the EWBs which operate in a floating type or a sliding type, the differential gear type EMBs pull an outer pad in a direction toward a disc from the beginning, and thus have a response time faster than those of other EMBs having the floating type or the sliding type. Here, an operation sequence of a floating or sliding caliper is as follows. First, (1) an inner pad is pushed toward a disc, (2) the disc is brought in contact with the inner pad, and then, the caliper moves back to pull an outer pad toward the disc, and (3) the outer pad is brought in contact with the disc, and then, there are compression deformations of driving parts and a bending deformation of the caliper occurs until a required clamping force is generated.
In the direct-clamping type EMBs that push a pad without a special self-reinforcing mechanism, it is important to select a reduction ratio of a reducer and a motor, which are essential elements of driving parts, at a development stage for maintaining a fast response time. Also, since the driving parts should be disposed in a space occupied by a hydraulic caliper, a selection of the driving parts are limited in designing the caliper. In order to solve this limit, there have been some approaches which develop a dedicated motor having a high power and a high torque for EMBs, and thus, the driving parts having the required performance is disposed in a predetermined space in a direct drive type without a reducer. However, in developing the dedicated motor, it is required the high cost and much time for the developed motor to verify the performance and reliability of it.
The EWBs generate a higher braking force than that of the direct-clamping type EMBs by a combination of a motor having a low power and a low torque and a reducer having a low reduction gear ratio due to a self-reinforcement based on a wedge structure. That is, in generating a clamping force for pushing a pad, the EWBs are higher in clamping and clamping efficiency than the EMBs, wherein the clamping efficiency means a ratio of a generated clamping force to an input force of driving parts.
However, the self-reinforcing effect depends on a wedge angle which is initially determined and is reflected in manufacturing, and it is unable to maintain a high braking efficiency under an actual condition that a frictional coefficient between a pad and a disc is changed.